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  <div class="question_difficulty">
   难度：Medium
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  <div>
   <h1 class="question_title">
    134. Gas Station
   </h1>
   <p>
    There are
    <em>
     N
    </em>
    gas stations along a circular route, where the amount of gas at station
    <em>
     i
    </em>
    is
    <code>
     gas[i]
    </code>
    .
   </p>
   <p>
    You have a car with an unlimited gas tank and it costs
    <code>
     cost[i]
    </code>
    of gas to travel from station
    <em>
     i
    </em>
    to its next station (
    <em>
     i
    </em>
    +1). You begin the journey with an empty tank at one of the gas stations.
   </p>
   <p>
    Return the starting gas station's index if you can travel around the circuit once in the clockwise direction, otherwise return -1.
   </p>
   <p>
    <strong>
     Note:
    </strong>
   </p>
   <ul>
    <li>
     If there exists a&nbsp;solution, it is guaranteed to be unique.
    </li>
    <li>
     Both input arrays are non-empty and have the same length.
    </li>
    <li>
     Each element in the input arrays is a non-negative integer.
    </li>
   </ul>
   <p>
    <strong>
     Example 1:
    </strong>
   </p>
   <pre>
<strong>Input:</strong> 
gas  = [1,2,3,4,5]
cost = [3,4,5,1,2]

<strong>Output:</strong> 3

<strong>Explanation:
</strong>Start at station 3 (index 3) and fill up with 4 unit of gas. Your tank = 0 + 4 = 4
Travel to station 4. Your tank = 4 - 1 + 5 = 8
Travel to station 0. Your tank = 8 - 2 + 1 = 7
Travel to station 1. Your tank = 7 - 3 + 2 = 6
Travel to station 2. Your tank = 6 - 4 + 3 = 5
Travel to station 3. The cost is 5. Your gas is just enough to travel back to station 3.
Therefore, return 3 as the starting index.
</pre>
   <p>
    <strong>
     Example 2:
    </strong>
   </p>
   <pre>
<strong>Input:</strong> 
gas  = [2,3,4]
cost = [3,4,3]

<strong>Output:</strong> -1

<strong>Explanation:
</strong>You can't start at station 0 or 1, as there is not enough gas to travel to the next station.
Let's start at station 2 and fill up with 4 unit of gas. Your tank = 0 + 4 = 4
Travel to station 0. Your tank = 4 - 3 + 2 = 3
Travel to station 1. Your tank = 3 - 3 + 3 = 3
You cannot travel back to station 2, as it requires 4 unit of gas but you only have 3.
Therefore, you can't travel around the circuit once no matter where you start.
</pre>
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  <div>
   <h1 class="question_title">
    134. 加油站
   </h1>
   <p>
    在一条环路上有&nbsp;
    <em>
     N
    </em>
    &nbsp;个加油站，其中第&nbsp;
    <em>
     i
    </em>
    &nbsp;个加油站有汽油&nbsp;
    <code>
     gas[i]
    </code>
    <em>
     &nbsp;
    </em>
    升。
   </p>
   <p>
    你有一辆油箱容量无限的的汽车，从第
    <em>
     i
    </em>
    个加油站开往第
    <em>
     i+1&nbsp;
    </em>
    个加油站需要消耗汽油&nbsp;
    <code>
     cost[i]
    </code>
    <em>
     &nbsp;
    </em>
    升。你从其中的一个加油站出发，开始时油箱为空。
   </p>
   <p>
    如果你可以绕环路行驶一周，则返回出发时加油站的编号，否则返回 -1。
   </p>
   <p>
    <strong>
     说明:
    </strong>
    &nbsp;
   </p>
   <ul>
    <li>
     如果题目有解，该答案即为唯一答案。
    </li>
    <li>
     输入数组均为非空数组，且长度相同。
    </li>
    <li>
     输入数组中的元素均为非负数。
    </li>
   </ul>
   <p>
    <strong>
     示例&nbsp;1:
    </strong>
   </p>
   <pre><strong>输入:</strong> 
gas  = [1,2,3,4,5]
cost = [3,4,5,1,2]

<strong>输出:</strong> 3

<strong>解释:
</strong>从 3 号加油站(索引为 3 处)出发，可获得 4 升汽油。此时油箱有 = 0 + 4 = 4 升汽油
开往 4 号加油站，此时油箱有 4 - 1 + 5 = 8 升汽油
开往 0 号加油站，此时油箱有 8 - 2 + 1 = 7 升汽油
开往 1 号加油站，此时油箱有 7 - 3 + 2 = 6 升汽油
开往 2 号加油站，此时油箱有 6 - 4 + 3 = 5 升汽油
开往 3 号加油站，你需要消耗 5 升汽油，正好足够你返回到 3 号加油站。
因此，3 可为起始索引。</pre>
   <p>
    <strong>
     示例 2:
    </strong>
   </p>
   <pre><strong>输入:</strong> 
gas  = [2,3,4]
cost = [3,4,3]

<strong>输出:</strong> -1

<strong>解释:
</strong>你不能从 0 号或 1 号加油站出发，因为没有足够的汽油可以让你行驶到下一个加油站。
我们从 2 号加油站出发，可以获得 4 升汽油。 此时油箱有 = 0 + 4 = 4 升汽油
开往 0 号加油站，此时油箱有 4 - 3 + 2 = 3 升汽油
开往 1 号加油站，此时油箱有 3 - 3 + 3 = 3 升汽油
你无法返回 2 号加油站，因为返程需要消耗 4 升汽油，但是你的油箱只有 3 升汽油。
因此，无论怎样，你都不可能绕环路行驶一周。</pre>
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